Did you know that the 112-inch-fan Pratt & Whitney PW4000 engine used on the Boeing 777–200/-200ER/-300 have hollow fan blades?
Why you might ask? According to Pratt & Whitney “Using hollow titanium, shroudless fan blades, the PW4000 provides high efficiency and low noise along with superb resistance to foreign object damage”.
The fan blades have “a hollow core, wide chord airfoil made of a titanium alloy with 6 percent vanadium and 4 percent aluminum as alloying elements”.
The advantage to having a wide chord engine is that there are fewer, wider, blades meaning a wide chord fan engine will have fewer, but more effective, fanblades compared to a regular jet engine.
Since these blades are made larger they are heavier, which is why it is normal for the blades to be made hollow to reduce weight.
You may have heard about this particular engines type from the B777-200 incidents back in February regarding the fan blades on their variants that used the PW4000 engines. You can see a close-up inside look of a broken hollow engine blade here, from the NTSB investigation into the incidents.
References: Pratt & Whitney